It may not be as risky as you think to allow your sellers an out.
On your next listing presentation you may be competing against an agent or broker who is offering a warranty to the prospect. You should at least know how this works, and maybe you can give it a try also.
Any contract is usually a bit intimidating to people, and when listing their home for sale, it's a big step committing to six months or more with a real estate company. Questions are running through the prospects' minds about how well you'll follow through, if you'll advertise like you say and many others.
You can set them at ease, and get an advantage toward getting the listing by offering an opt-out warranty at any time if they're not satisfied with your performance. A big risk? Perhaps it is. However, do you really want to work with someone who doesn't want to work with you?
And what about all that up-front time and expense in getting the photos, placing the sign, gathering documents, etc. Here's where you can help yourself with a logical buy-out for the client. They should understand the time and expense if you outline the work items for them. Then just tell them that your opt-out warranty is available at any time during the listing with a $??? fee for cancellation to reimburse you for the time and expenses. You calculate the number. You really don't need to get into complicated spreadsheets of what you've done for any particular client. Just come up with a number.
You'll want to get the approval of your broker, and you might be able to get a better split on this amount if you're doing all the work. You will probably find that you'll rarely have this clause exercised, but you should find that your listing closings will be easier.